This invention relates to apparatus for starting and operating a plurality of electric discharge lamps, such as fluorescent lamps or the like. More particularly, the invention is directed to such apparatus which is more reliable than similar prior art apparatus in that it prevents burn-out of the ballast transformer windings in the case of a malfunction of a discharge lamp.
U.S. Pat. No. 2,558,293 (A. E. Feinberg, 6/26/51) describes a ballast apparatus for starting and operating two gas discharge lamps in series with sequential starting of the lamps. That ballast comprises a three-winding transformer having a primary winding P, a first secondary winding S.sub.1 and a second secondary winding S.sub.2, all of the winding being connected serially in the order named with the secondary windings arranged in voltage bucking relationship to one another. The windings are all mounted on a unitary elongate magnetic core side by side, with the two secondary windings on opposite sides of the primary winding. A magnetic shunt is provided between the first (start) winding designated S.sub.1 and the primary winding P. The primary winding P and the first (start) winding S.sub.1 are loosely coupled which provides a relatively high leakage reactance. A first discharge lamp is connected in series with a capacitor across the series connection of the primary winding P and the start winding S.sub.1. The second lamp is connected across the series connection of the first (S.sub.1) and second (S.sub.2) secondary windings. FIG. 4 of the patent shows a variation thereof and other variations are also possible. In general, one gas discharge lamp is connected across windings excluding the second secondary S.sub.2 and including at least the first secondary S.sub.1, and a second gas discharge lamp is connected across windings which include at least both of the secondary windings.
In operation, when the primary winding P is energized by the AC supply voltage, a voltage will be produced in both the primary winding P and the first secondary winding S.sub.1 which will be sufficient to ignite the first gas discharge lamp. As a result, current will flow through the start winding S.sub.1 and, because of its high leakage reactance, a voltage will be produced therein of a phase such as to produce a substantial voltage component additive relative to the voltage induced in the second secondary winding S.sub.2. Thus, the second gas discharge lamp will now ignite. With both of the discharge lamps operating, there will be a series path for the major portion of the current through the lamps and the second secondary winding S.sub.2. The first secondary winding S.sub.1 is effectively bypassed because its high leakage reactance impedes the flow of current therethrough. Therefore, the winding S.sub.1 can be, and in commercial versions has been made of a large number of turns of very fine wire since it carries so little current during operation. The two lamps are ignited in sequence and thereafter are operated in series from the AC supply voltage via the secondary winding S.sub.2 and the aforesaid capacitor. The resultant ballast is very small and compact, provides high efficiency and high power factor operation, and generates very high lamp ignition voltages with relatively little copper.
Although ballast devices designed in accordance with the above-described U.S. patent performed successfully for many years, a problem occurred after a long period of lamp operation. The second to start lamp lost emission material from one of its cathodes so that it then operated as a rectifying tube. In that case a rectified current flowed in the circuit, essentially a pulsed DC current. This current could not pass through the series capacitor and consequently was forced to flow through the start winding S.sub.1. The amplitude of current that flowed in the start winding was much higher than the current for which this winding was designed. Since the start winding was designed to withstand relatively low currents, it would either heat up excessively or burn out. The ballast would then have to be replaced at considerable expense and inconvenience.
In order to solve this problem, Feinberg invented a ballast apparatus which issued as U.S. Pat. No. 2,682,014 (6/22/54). This patent proposed to add a second capacitor C.sub.2 connected in series with the second secondary winding S.sub.2 in order to prevent the flow of rectified (DC) current through the second discharge lamp. This ballast circuit therefore provided a first capacitor C.sub.1 in series with the first lamp and a second capacitor C.sub.2 in series with the second lamp (and also in series with the second secondary winding S.sub.2). Each of these capacitors had a capacitance value of approximately twice the capacitance value of the single series capacitor of the earlier Feinberg patent (U.S. Pat. No. 2,558,293).
In the event that the second lamp became a rectifier, the two capacitor ballast circuit was quite effective in preventing the flow of DC current in each of the lamps since each lamp now had a capacitor connected in series therewith to block any DC current flow therein. The flow of rectifying current through the secondary windings also was blocked, thus protecting the start winding S.sub.1 from burn-out. Although the two capacitor ballast circuit was effective in protecting the transformer secondary winding (S.sub.1) from failure, the resultant apparatus was too expensive to compete against other commercial ballast devices. In addition, the two capacitor ballast produced an unacceptable difference in the current and power the two capacitor ballast avoided the problems associated with lamp rectification operation, it did not provide a practical and commerically competitive apparatus.
A further attempt to cure the problems associated with the one and two capacitor ballast devices described above resulted in a delta arrangement of three capacitors described in U.S. Pat. No. 3,198,983 (8/3/65), also in the name of A. E. Feinberg. The capacitance values were then chosen so that the normal operating current still flowed through each of the lamps and without a material change in the existing core lamination or the transformer windings. Once again, although the three-capacitor ballast provided the required ballast protection, it also was too expensive for widespread commercial use.
In recent years a new form of lamp has come into widespread use, the so-called energy saver lamp. A characteristic of this lamp is that it is more susceptible to a loss of cathode emissive material and therefore to the lamp rectification problem described above. It has been found that in the case of a 60 watt energy saver lamp the DC current that flows in the event of lamp rectification is greater than that present with a standard 75 watt lamp. As a consequence, there have been more ballast failures with energy saver lamps than was previously experienced with the conventional discharge lamps.
A first attempt to solve the lamp rectification problem in energy saver lamps utilized the systems shown in U.S. Pat. No. 2,682,014, but the results were unsatisfactory because the starting currents proved to be too low to reliably ignite the lamps. In addition, there was a considerable imbalance in the currents between the first and second discharge lamps, which of course is undesirable from a lighting standpoint and to meet the standards required for lamp output.